1. Field of the Invention
The present invention generally relates to an interface for a high performance graphics adapter, and more particularly, to an apparatus and method for interfacing a computer system to a high performance graphics adapter and for providing compatibility for the graphics adapter with other, more commonly available, graphics formats.
2. Description of the Prior Art
In a conventional computer system, such as an "IBM compatible" microcomputer, graphics adapters are provided to serve as an interface between an application program operating on the computer system and a visual output medium, such as a cathode ray tulle (CRT) or flat panel display. Examples of such graphics adapters currently in widespread use are those that utilize various types of graphics formats, such as EGA, VGA, MCGA, Hercules, 8514/A, etc. Many of these types of graphics adapters have become de facto industry standards.
For example, VGA graphics adapters are used extensively by microcomputers to provide reasonably high resolution multi-color graphics on a compatible CRT monitor. The VGA standard, as normally implemented, allows up to 16 different colors to be displayed simultaneously from a palette of over 256,000 different color combinations, and at a resolution of 640 pixels horizontally and 480 vertically. At lower resolutions, an even greater number of colors may be displayed simultaneously. In addition, some extensions to the VGA standard allow for a resolution beyond the standard 640 by 480 normal resolution.
Because of the relatively high resolution and color flexibility found in the VGA standard, this and other similar standards, such as EGA, have become very popular among computer owners seeking high performance graphics at reasonable prices. However, as is common with other aspects 17 computer technology, significant improvements have been made in recent years with respect to graphics adapters. Notable among these improvements is the introduction of advanced, high-speed dedicated graphics system processors (GSPs), capable of providing advanced graphics capabilities at high speeds. These GSPs provide in a minimal number of integrated circuit packages many types of advanced graphics capabilities previously only achievable by utilizing multiple integrated circuit packages or emulating such functions in software. Moreover, these GSPs are able to provide such functionality at speeds greatly exceeding what was previously possible.
One such example of a GSP is a member of the TMS340.times.0 family of graphics system processors available from Texas Instruments of Dallas, Texas (collectively referred to as the "340.times.0"), and its regional offices. Specific examples of this processor family include the 34010 first generation processor and the 34020 second generation processor. The 340.times.0 family is an advanced, 32-bit internal data path microprocessor, optimized for graphics display systems, and capable of being used with the Texas Instruments Graphics Architecture (TIGA), a software interface that standardizes communication between application software and all TMS340.times.0-based hardware for "IBM-compatible" personal computers. The capabilities of the 340.times.0 represent a major improvement in speed and power over previously existing graphics processors, including VGA graphics processors.
While the use of the 340.times.0 GSP may provide many advantages over graphics adapters such as those utilizing the VGA standard, the 340.times.0 is unfortunately incompatible with the previous graphics standards (such as VGA) from an addressing and data format point of view. Specifically, VGA uses a special addressing scheme developed by IBM, while the 340.times.0 uses a conventional linear addressing scheme. In addition, VGA uses a special video data storage scheme developed by IBM to store information about each picture element (pixel), while the 340.times.0 uses a more conventional "packed pixel" format whereby each piece of pixel information is "packed" next to one another. Thus, the 340.times.0 GSP is unable to effectively operate upon graphics data generated by an application computer program operating on an associated host microprocessor when the graphics data is designed for use with a VGA graphics adapter.
To circumvent this limitation, several display adapters have been developed recently which allow for the alternative use of the advanced capabilities of GSPs, such as the 340.times.0 processor, while including additional redundant circuitry to support more standard graphics standards such as VGA. For example, some of these adapters are based on the 340.times.0 processor as well as an on-board VGA chip set. The user may manually select whether the 340.times.0 GSP will be utilized (via TIGA) or whether the VGA circuitry will be utilized through the use of a software setup utility.
In another arrangement, display adapters have previously been developed utilizing both 340.times.0 circuitry as well as VGA circuitry which may automatically utilize the appropriate circuitry depending upon what type of data is received. In this case, if TIGA data is received by the adapter, the 340.times.0 circuitry is used. On the other hand, if VGA data is received, this data is "passed through" to the VGA circuitry.
In both cases described above, because the 340.times.0 circuitry and the VGA circuitry operate wholly independently from one another, redundant circuitry is needed when both the 340.times.0 GSP and VGA circuitry are designed to be implemented within the same display adapter. Similar functions common to both types of formats are repeated for both the 340.times.0 and VGA. For example, a CRT controller, a sequencer, etc. are needed in both environments, and these components must therefore be duplicated in different forms for each of the 340.times.0 and VGA systems. This contributes to higher costs for manufacturing such adapters.
Therefore, it would clearly be advantageous to eliminate the inherent redundancy in having a complete 340.times.0 system as well as a complete VGA system located within the same display adapter. As described below, the present invention effectively addresses this problem.